In commercially available flat cables comprising fused portions and twisted pair portions, the fused portions to be connected to a connector are disposed at equal spacings with each of the fused portions being sandwiched between the twisted pair portions.
In the twisted pair portions in this type of flat cables, adjacent twisted pairs are twisted in directions opposite to each other from the viewpoint of preventing crosstalk. When attention is drawn to one twisted pair, however, in some of this type of flat cables, the twisted pair portions sandwiching the fused portion are twisted in directions opposite to each other, while in the other flat cables of this type, the twisted pair portions sandwiching the fused portion are always twisted in the same direction. This difference is attributable to a difference in the production system.
An example of the construction of the system for producing flat cables will be explained.
A system for producing a flat cable comprises: a feeder for delivering insulated conductors for a flat cable at even tension; a twister for twisting two insulated conductors to each other to form a twisted pair; a fusing device for fusing all the insulated conductors to one another to form a fused portion with the insulated conductors being juxtaposed to each other; a take-up unit for taking up at a constant speed insulated conductors that have been subjected to pair twisting and fusing; a coiler for winding the cable around a bobbin; a controller for controlling the operation and stopping of the twister and for controlling the operation and stopping of the fusing device; and the like.
In the flat cable wherein the twisted pair portions sandwiching the fused portion are twisted in directions opposite to each other, during the formation of a certain twisted pair portion, twist is accumulated in the insulated conductors between the twister and the feeder. Since, however, twisting is carried out in the opposite direction in the formation of the next twisted pair portion, the accumulation of the twist is eliminated. On the other hand, in the flat cable wherein the twisted pair portions sandwiching the fused portion are always twisted in the same direction, the twist between the twister and the feeder is increasingly accumulated. The twist cannot be infinitely accumulated in the insulated conductors between the twister and the feeder. Therefore, a twist back mechanism for eliminating the twist should be provided at the delivery portion.
The twist back mechanism should be provided in number equal to the number of pairs. In some of actual flat cables, the number of pairs is 50 or 100. This incurs very high cost for the provision of the twist back mechanisms.
The conventional flat cables described above are produced on the premise that the mounting spacings of the connector are constant. In recent years, however, there is a demand that the connector is provided at different mounting spacings in one cable, for example, at mounting spacings of 125 mm, 250 mm, 325 mm and the like. In order to meet the above demand, when a cable with the fused portions being provided at spacings of 125 mm and 250 mm is taken by way of example, if the length of the fused portions is 50 mm with the pitch of the twisted pairs being 25 mm, the number of twists is 3 in the portion having a fused portion spacing of 125 mm and 8 in the portion having a fused portion spacing of 250 mm. In the production of a cable wherein the portion with the number of twists being 3 is provided alternately with the portion with the number of twists being 8, when the twist sense is changed for the formation of each of the fused portions as described above, twist is increasingly accumulated between the feeder and the twister. This ultimately requires the provision of twist back mechanisms.